1. Field of the Invention
This invention relates to an airbag made at least partially from two panels, e.g., front and back panels, of relatively inelastic plastic film in such a manner that it automatically attains a desired shape without the use of tethers which connect the front and back panels.
2. Description of the Prior Art
A conventional driver side airbag (also referred to herein as a driver airbag) is made from pieces of either Nylon or polyester fabric which are joined together by sewing. The airbag is coated on the inside with neoprene or silicone for the purposes of (i) capturing hot particles emitted by the inflator and preventing holes from being burned in the fabric, and (ii) sealing the airbag to minimize the leakage of an inflating gas through the fabric. These airbags are conventionally made by first cutting two approximately circular sections of a material having a coating on only one side which will form a front panel and a back panel, and sewing them together with the coated side facing out. The back panel contains a hole for attachment to an inflator. Fabric straps, called tethers, are then sewn to the front panel. Afterwards, the airbag is turned inside out by pulling the fabric assembly through the inflator attachment hole placing the coated side on the inside. Assembly is completed by sewing the tethers to the back panel adjacent the inflator attachment hole.
If a conventional driver airbag is inflated without the use of tethers, the airbag will take an approximately spherical shape. Such an airbag would protrude significantly into the passenger compartment from the steering wheel and, in most cases, impact and injure the driver. To prevent this possible injury, the tethers are attached to the front and rear panels of the airbag to restrict the displacement of the front panel relative to the back panel. The result of the addition of such tethers is an airbag which has the shape of a flat ellipsoid with a ratio of the thickness of the airbag to its diameter of approximately 6. In the conventional airbag, the tethers are needed since the threads which make up the airbag fabric are capable of moving slightly relative to each other. The airbag is elastic for stresses which are not aligned with the warp or woof of the fabric. As a result, the fabric distorts to form an approximate sphere.
Moreover, the above-mentioned method of manufacturing an airbag involves a great deal of sewing and thus is highly labor intensive and, as a result, a large percentage of all driver airbags are manufactured in low labor cost countries such as Mexico.
Many people are now being injured and some killed by interaction with the deploying airbag. One of the key advantages of the film airbag described in the above referenced patent application is that, because of its much lower mass, the injury caused by this interaction is substantially reduced. In accordance with the teachings of that patent application, the driver airbag system can be designed to permit some significant interaction with the driver. In other words, the film airbag can be safely designed to intrude substantially further into the passenger compartment without fear of injuring the driver. Nevertheless, in some cases it may be desirable to combine the properties of a film airbag which automatically attains the proper driver airbag shape with a fabric. In such cases, interaction with the driver needs to be minimized.
Airbags made of plastic film are disclosed in the copending patent application Ser. No. 08/247,763 referenced above and incorporated herein by reference. Many films have the property that they are quite inelastic under typical stresses associated with an airbag deployment. If an airbag is made from flat circular sections of such films and inflated, instead of forming a spherical shape, it automatically forms the flat ellipsoidal shape required for driver airbags. This unexpected result vastly simplifies the manufacturing process for driver airbags since tethers are not required. Furthermore, since the airbag can be made by heat sealing two flat circular sections together without the need for tethers, the entire airbag can be made without sewing reducing labor and production costs. In fact, the removal of the requirement for tethers permits the airbag to be made by a blow molding or other similar process. Indeed, this greatly reduces the cost of manufacturing driver airbags.
In addition to the above referenced patent application, film material for use in making airbags is described in U.S. Pat. No. 4,963,412 to Kokeguchi. The film airbag material described in the Kokeguchi patent is considerably different in concept from that disclosed in the above referenced patent application Ser. No. 08/247,763 or the instant invention. The prime feature of the Kokeguchi patent is that the edge tear resistance of the airbag film material can be increased through the use of holes in the plastic films. Adding holes, however, reduces the tensile strength of the material by factor of two or more due to the stress concentration effects of the hole. It also reduces the amount of available material to resist the stress. As such, it is noteworthy that the Kokeguchi airbag is only slightly thinner than the conventional fabric airbag (320 micrometers vs. the conventional 400 micrometers) and is likely to be as heavy or perhaps heavier than the conventional airbag. Also, Kokeguchi does not disclose any particular shapes of film airbags. As will be discussed below in detail, the airbags constructed in accordance with the present teachings attain particular shapes based on the use of the inelastic properties of particular film materials.
The neoprene or silicone coating on conventional driver airbags, as mentioned above, serves to trap hot particles which are emitted from the inflator, such as a conventional sodium azide inflator. A film airbag will be vulnerable to such particles and as a result will not work effectively with sodium azide inflators. Fortunately, new inflators are being developed which do not produce hot particles but instead produce gases which are substantially cooler than gases produced by sodium azide inflators.
In the conventional airbag, the propellant which is used to inflate the airbag also is used to force open a hole in the vehicle trim, called the deployment door, permitting the airbag to deploy. Since the mass of a film airbag is substantially less than the conventional fabric airbag, much less energy is required to deploy the airbag in time. However, substantial pressure is still required to open the deployment door. Also, if the pressure now used to open the deployment door is used with film airbags, the rate of deployment, once the door has been opened, will be substantially faster than conventional airbags. This rapid deployment puts excessive stresses on the film airbag and increases the chance that the occupant will be injured thereby. For most implementations of the film airbag, an alternate less energetic method of opening the deployment door is necessary.
One such system is disclosed in Barnes et al. U.S. Pat. No. 5,390,950 titled "Method and arrangement for forming an air bag deployment opening in an auto interior trim piece". This patent describes a method of forming an air bag deployment opening in an interior trim piece having an vinyl skin overlying a rigid substrate so as to be invisible prior to operation of the air bag system and comprises an energy generating linear cutting element arranged in a door pattern beneath the skin acting to degrade or cut the skin when activated.
The goal of the Barnes et al. patent is to create an invisible seam when the deployment door is located in a visible interior trim panel. This permits greater freedom for the vehicle interior designer to create the particular aesthetic effect that he or she desires. The invisible seam of the Barnes et al. patent is thus created for aesthetic purposes with no thought toward any advantages it might have to reduce occupant injury or advantages for use with a film airbag, or to reduce injuries at all for that matter. One unexpected result of applying the teachings of this patent therefore, is that the pressure required to open the deployment door is substantially reduced. When used in conjunction with a film airbag, this result is important since the inflator can be designed to provide only sufficient energy to deploy and inflate the very light film airbag thereby significantly reducing the size of the inflator. The additional energy required to open a conventional deployment door above that required to open a door constructed in accordance with the teachings of the Barnes et al. patent is not required within the inflator. Furthermore, since a film is more vulnerable to being injured by ragged edges on the deployment door than a conventional fabric airbag, the device of the Barnes et al. patent can be used to pyrotechnically cut open the deployment door permitting it to be easily displaced from the path of the deploying airbag, minimizing the force of the airbag against the door and thus minimizing the damage to the film airbag from the deployment door. Since Barnes et al. did not contemplate a film airbag, advantages of its use with the pyrotechnically opening deployment door could not have been contemplated.
The discussion of the self-shaping airbag thus far has been limited to film airbags. An alternate approach is to make an airbag from a combination of fabric and film. The fabric provides the tear resistance and conventional airbag appearance. The film forces the airbag to acquire the flat ellipsoidal shape desired for driver airbags without the use of tethers and permits the airbag to be assembled without sewing using heat and/or adhesive sealing techniques. Such a hybrid airbag is made from fabric and film which have been laminated together prior to the cutting operation. Naturally, the combination of a film and net, as described in the above referenced patent application, is equally applicable for the airbag described here and both will be referred to herein as hybrid airbags.
A finite element analysis of conventional driver side airbags shows that the distribution of stresses is highly unequal. Substantial improvements in conventional airbag designs can be made by redesigning the fabric panels so that the stresses are more equalized. Today, conventional airbags are designed based on the strength required to support the maximum stress regardless of where that stress occurs. The entire airbag must then be made of the same thickness material as that chosen to withstand maximum stress condition. Naturally, this is wasteful of material and attempts have been made to redesign the airbag to more closely equalize the stress distribution and thus permit a reduction in fabric strength and thus thickness and weight. However, this optimization process when used with conventional fabric airbags can lead to more complicated assembly and sewing operations. Thus, there is a tradeoff between manufacturing cost and airbag optimization.
With the film airbag manufactured using blow molding techniques, for example, much greater freedom is permitted to optimize the airbag vis-a-vis equalization of the stress. First, other than tooling cost, the manufacturing cost of an optimized airbag is no greater than for a non-optimized airbag. Furthermore, the thickness of the film can be varied from one part of the airbag to another to permit the airbag to be thicker where the stresses are greater and thinner where the stresses are less. A further advantage of blow molding is that the film can be made of a single constituent material. When the airbag is fabricated from sheet material, the outside layer of the material needs to be heat sealable such as polyethylene or else a special adhesive layer is required where the sealing occurs.
One example of an inflatable film product which illustrates the technology of this invention is the common balloon made from metalized "Mylar" plastic film found in many stores. Frequently these balloons are filled with helium. They are made by heat sealing two flat pieces of film together as described in U.S. Pat. Nos. 5,188,558, 5,248,275, 5,279,873, and 5,295,892. The shape of these balloons, which is circular in one plane and elliptical in the other two planes, is very nearly the shape which is desired for a driver side airbag. This shape is created when the pressure within the balloon is sufficiently low such that the stresses induced into the film are much smaller than the stresses needed to significantly stretch the film. The film used is relatively rigid and has difficulty adjusting to form a spherical shape. In contrast, the same airbag made from woven material more easily assumes an approximate spherical shape requiring the use of tethers to create the shape which comes naturally with the Mylar balloons.
One problem with film balloons is that when a hole is punctured in the balloon it fails catastrophically. One solution to this problem is to use the combination of a film and net as described in the above referenced patent application. Such materials have been perfected for use as sail material for lightweight high performance sails for sailboats. One example is marketed under the trade name Bainbridge Sailcloth SL Series.TM., and in particular SL 500-P.TM., 1.5 mill. This material in a laminate of a film and a net. Such materials are designed to permit heat sealing thereby eliminating threads and the stress concentrations associated therewith. Heat sealing also simplifies the manufacturing process for making sails. Another preferable solution is to make the airbags from a film material which naturally resists tears, that is, one which is chemically formulated to arrest a tear which begins from a hole, for example.
Applications for the self shaping airbag described herein include all airbags within the vehicle which would otherwise required tethers or complicated manufacturing from several separate panels. Most of these applications are more difficult to solve or unsolvable using conventional sewing technology. The invention described herein solves the above problems by using the inelastic properties of film, otherwise stated as their high modulus of elasticity, plus innovative designs based on analysis including mathematical modeling plus experimentation. In this manner, the problems discussed above, as well as many others, are alleviated or solved by the self shaping airbags described in the paragraphs below.